Usually environmental legislation in developed countries sets maximum levels for one or more of the following pollutants in exhaust gases: solid particle levels, CO, NOx, and volatile organic substances. Also, limits on the temperature of the gases in exhaust chimneys and opacity of the gases are often specified in environmental legislation. However legislation in the various countries is not uniform and the permitted levels of pollutants vary from country to country.
Basically, two types of legislation can be distinguished: those which limit the NOx content of the exhaust gases, and those which do not. Spain is an example of a country in which the NOx content of the exhaust gases is restricted. For example, in Spain, exhaust gases from coffee roasters may contain a maximum: solids particle content of 150 mg/N m.sup.3, CO content of 500 ppm, NOx content of 300 ppm, and an opacity level of 2. However there is no limit on the total organic carbon (TOC) that the exhaust gases may contain. Germany, however, is an example of a country in which the NOx content of the exhaust gases is not restricted. For example, in Germany, exhaust gases from coffee roasters may contain a maximum: solids particle content of 150 mg/N m.sup.3, CO content of 50 mg/N m.sup.3, TOC content of 50 mg/N m.sup.3. However there is no limit on the NOx content of the exhaust gases.
Traditionally coffee roasters comprise a fuel burner (the fuel usually being natural gas or oil) which receives and heats air introduced from the ambient. The heated air, mixed with the combusted gas, is usually at a temperature of about 370 to 450.degree. C. and is directed to a roasting chamber, usually in the form of a rotary cylinder or tumbler, which contains the green coffee beans. The heated air and combusted gases then roast the coffee beans. The exhaust gases from the roasting chamber are usually at a temperature of about 150.degree. C. They are usually directed to a cyclone separator to remove entrained solid particles and are then vented; often without treatment. With this basic system, the pollutant levels in the exhaust gases will very often exceed the levels set in the relevant legislation.
Consequently, numerous systems have been proposed to reduce and control the levels of pollutants in the exhaust gases. One proposal is to direct only a part of the mixture of hot air and combusted gases from the combustion chamber into the roasting chamber; the remainder being directed to a cyclone separator and then vented. The portion of the hot air and combusted gases which is directed to the roasting chamber is passed through a cyclone separator once leaving the roasting chamber and recycled to the combustion chamber. However, upon termination of roasting by quenching with water, large amounts of steam are produced. This causes a large increase in the volume of exhaust gases, which increases the pressure and volume of the exhaust gases within the recycle circuit. This in turn can cause chilling of the flame of the burner, with the result that the quality of the combusted gases deteriorates. Also, the pollutant levels in the exhaust gases which are vented are high.
To reduce the pollutant levels in the exhaust gases which are vented, it has been suggested to include a secondary burner into the system prior to venting. Although this does reduce pollutant levels to some extent, the levels are still high. Also, the temperature in the secondary burner needs to be very high, for example above about 700.degree. C., to obtain significant reduction in pollutant levels. Even then CO levels are usually not reduced enough. Also energy consumption is high.
It has also been suggested to include a catalytic converter into the system prior to venting of the exhaust gases. The use of a catalytic converter is the most appropriate solution in countries where the legislation does not place any limit on NOx content since the use of catalytic converters results in production of NOx. Hence, although the catalytic converter reduces CO levels, it actually increases NOx levels. However, it is difficult to introduce a catalytic converter into the system after the secondary burner due to the high temperature of operation of the secondary burner.